Question about which vacum advance port

[tntempest]

I have a 79 coupe with a newer roller motor that is for sale over in the classifieds, anyway which port does the vacuum advance go on an early style distributor I guess a 83-85 vintage. Does it go to the ported vacuum on a Holley, the port on the side that does not give vacuum until above idle or does it go to the lower one that gives full manifold vacuum. It seems to advance the timing at idle on the full manifold vacuum and I was wondering if that is correct.

[JACook]

The factory routed it through a thermal vacuum switch that put manifold vacuum
on it when the engine is cold, and ported vacuum once the engine warms up.

If you're not using the thermal vacuum switch, I would use the ported vacuum.

[gdniel]

I used ported vacuum on my 81 also.

[tntempest]

Well I did a search and there was a big debate in January, I will read over that and make my decision. I just thought it was odd for it to advance at idle, whats the point of base timing then.

[donutbandit]

The proper way to do it is with ported vacuum. In that thread in January, some people maintained it was manifold vacuum, but they had done extensive reworking of their distibutors.

[ujslost]

ive never touched the insides of my dist, its a reman 85 unit from the parts store. I run my vac on the manifold vac. tried the ported vac. and the manifold seem to do much better. just my experience.

[JACook]

Seemed to do much better how? And might this not have something to do with the other problem
you're having right now?

[donutbandit]

I'm not going to get into thids long, lengthy discussion again, as we just beat it to death a few months ago. If your car runs better hooked to manifold vac, then something is wrong with your base timing. Manifold vac pulls the advance OFF when you want it ON, and vice versa.

[JACook]

Len, it's not just about the base timing, it's also about the relationship between timing,
manifold vacuum, and throttle angle. Whenever I hear people say engines with big cams
like manifold vacuum on the distributor, I instinctively ask whether this isn't just masking
a problem where the engine wants to idle on the transfer slots with normal base timing.

There are better ways to deal with that problem...

[ScreeminChikin]

I'm not going to get into thids long, lengthy discussion again, as we just beat it to death a few months ago. If your car runs better hooked to manifold vac, then something is wrong with your base timing. Manifold vac pulls the advance OFF when you want it ON, and vice versa.

I'm also not going to get too deep into this because as has already been said, there are plenty of lengthly threads on this subject and I have voice my views in those already. However, manifold vacuum does not "pull the advance OFF when you want it ON". Vacuum adds advance...period. Under light load/high vacuum such as cruising and idle, advance is added. Lack of vacuum (like ported vacuum at low throttle angle) "pulls advance off". In actuality it doesnt pull it off, it just doesn't pull it in.

[JACook]

Ported vacuum adds advance as you tip into the throttle. Manifold vacuum does not.
Depending on how you have your advance can adjusted, it may even do exactly the
opposite. But best case, the timing stays the same.

[85stanggt]

Question: is ported at idle really 0?

From the factory, setting the timing at idle would require you to remove the advance line and plug it. This wouldn't be necessary if it was getting ported, right? (Unless of course there is some ported vacuum at idle).

I initially was going to check and make sure my car was hooked up right to ported because that's what I read until that debate which just left things up in the air for me. I'd like to maybe finally figure this out.

Also, JACook, which tree should the dist tubes be hooked to? From the schematic it looks like the tree with the electrical plug, so I switched it because my car was reversed (quite possibly by some mechanic).

[JACook]

The spark port sits above the throttle blade when it's closed. If everything is as it should
be, there will be no vacuum on the advance line at idle, after the engine is warm.

The reason you normally disconnect the hose when setting the timing is because there
can be vacuum on it, for a number of reasons. First, when the engine is cold, the thermal
vacuum switch puts manifold vacuum on the advance. Also, when the choke is set, the
throttle blade is opened enough to induce vacuum on the spark port. If for some reason,
such as a longer duration cam, the throttle is screwed open to compensate, the additional
throttle angle may be enough to induce vacuum on the spark port. I've also seen where
the thermal vacuum switch can develop an internal leak that will allow manifold vacuum
to bleed over to the distributor port.

And yes, the grey thermal vacuum switch with the electrical connector controls the
distributor advance. It should be in the second threaded hole in from the valve cover.
The green one that sits next to the driver side valve cover is for EGR.

[X-isles]

I'm also not going to get too deep into this because as has already been said, there are plenty of lengthly threads on this subject and I have voice my views in those already. However, manifold vacuum does not "pull the advance OFF when you want it ON". Vacuum adds advance...period. Under light load/high vacuum such as cruising and idle, advance is added. Lack of vacuum (like ported vacuum at low throttle angle) "pulls advance off". In actuality it doesnt pull it off, it just doesn't pull it in.


This is 100% correct... nothing more needs to be said.

[ujslost]

Seemed to do much better how? And might this not have something to do with the other problem
you're having right now?
seems to have more power like it advances quicker. I dunno, but its better. And the idle hanging up just started happening. Fixed it today. I needed a throttle return spring.

[ujslost]

so which is better manifold or the other?

[JACook]

so which is better manifold or the other?

Best thing I can suggest is to strap a vacuum gauge to each of the ports, and drive around,
so you know how each behaves. Then consider what is the purpose of vacuum advance in
the first place. Once you do that, you'll better understand what we're arguing about...

[ujslost]

one has vac at idle and the other doesnt, i know that already

[X-isles]

Most engines need advance at idle and at very light cruise. The only way to get advance under those conditions is manifold vac.

"Ported" vacuum is nothing more than a very early attempt to increase exhaust gas temp for emissions purposes.

[JACook]

one has vac at idle and the other doesnt, i know that already

That's a start. But it really helps to be able to see the actual vacuum under varying driving
conditions.

Simply put, manifold vacuum is inversely proportional to throttle angle. But ported vacuum
does not behave with any linear relationship to throttle angle. Ported vacuum follows a
narrow bell curve, that peaks at the point where the throttle blade is directly adjacent
to the port. That's usually ~10% throttle, or in other words, light cruise. Once you move
away from this angle, in either direction, ported vacuum decreases.

Ported vacuum's primary purposes are to improve light-throttle efficiency, by compensating
for the slow burn that happens with the combination of lean mixture and lower dynamic
compression ratio that happen at cruise. It also lowers part-throttle exhaust gas temps.
The other benefit is to reduce the tendency for the engine to stumble when tipping lightly
into the throttle. Manifold vacuum can be used instead for the former case, though there
are often unwanted side effects. But manifold vacuum cannot perform the second function.
Without ported vacuum, your only recourse is to throw more fuel at the problem. That
won't do fuel mileage any favors...

People seem to want to treat this as some kinda religious discussion. What I'm trying to do
is to get people to actually think about what these systems do, and why they exist. That's
why I suggest hooking up vacuum gauges and hitting the road. That picture will be worth
far more than just reading what someone else has to say.

[JACook]

Most engines need advance at idle and at very light cruise. The only way to get advance under those conditions is manifold vac.

"Ported" vacuum is nothing more than a very early attempt to increase exhaust gas temp for emissions purposes.

Sorry, you have it exactly wrong, on both counts.

[X-isles]

I would suggest that anyone who wants good solid info on manifold vs vac advance should read "Timing and Vacuum Advance 101". Its not hard to find, its basic and straight to the point. 3 pages of great info.

[donutbandit]

I kinda like what somebody said back in the old thread, the gist of which was:

Yeah, all those highly paid engineers back in the day who invented vacuum advance and the ported vacuum system didn't know beans. It took a bunch of shade tree yahoos to come along and show them they did it all wrong from the beginning. LOL.

That being said, I can see how it might SEEM to someone who has a problem like a sticking mechanical advance, incorrectly set static timing, or a huge cam, that the manifold vacuum works better. Engines with a huge cam make poor vacuum to begin with. Someone with a timing problem will experience a sudden surge when they step on the gas and all that pent up vacuum hits the pot. The surge will quickly turn flat as the vacuum bleeds off.

If hooking the vacuum advance to the manifold cures your idle problem, that indicates a timing problem to begin with. You are applying full advance at idle, when there is supposed to be none or very little. The second you step on the gas, your timing gets retarded rather than advanced, as the built up vacuum bleeds away.

[donutbandit]

I would suggest that anyone who wants good solid info on manifold vs vac advance should read "Timing and Vacuum Advance 101".

If that's where you got the misinformation you have been posting, it's not worth reading.

[JACook]

If that's where you got the misinformation you have been posting, it's not worth reading.

+1.

Rather than cross-post, here's a link to my response to "Vacuum Advance 101"...)
http://vb.foureyedpride.com/showthread.php?p=605175#post605175